Hydrophiloidea

Water scavenger beetles

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Introduction

The superfamily Hydrophiloidea (the water scavenger beetles) represents one of the most ecologically and morphologically diverse groups of Coleoptera with more than 3200 species broadly distributed in all biogeographic regions except Antarctica (Short & Hebauer 2006). As its name implies, a large proportion of hydrophiloids are aquatic. However, the group’s habits are in reality highly varied. While many live in ‘traditional’ freshwater habitats, such as ponds and stream margins, many occupy more specialized aquatic situations such as waterfalls (e.g. Oocyclus and allies), subterranean streams (e.g. Troglochares) and the water tanks of bromeliads (e.g. Phaenonotum). Other lineages have diversified in a staggering array of terrestrial niches: many are generalists of decaying vegetation or dung (e.g. Sphaeridium); some Oosternum are myrmecophilous; Tormissus is restricted to penguin guano, and several genera are even found in flowers. Although most larval hydrophilids occupy the same ecological niche as the imago, they are predaceous compared to the detritivore or algal-feeding adults.

Discussion of Phylogenetic Relationships

Constitution and higher-level relationships of the Hydrophiloidea

The first (and only) comprehensive analysis of the Hydrophiloidea using cladistic methods was undertaken by Hansen (1991). Based primarily on adult morphology, the resulting classification (here after, the “Hansen Classification”) has been widely adopted by nearly all aquatic Coleoptera systematists and further codified in a world catalogue of species (Hansen 1999). The increased accessibility to literature and testable hypotheses made possible by Hansen’s landmark works reinvigorated interest in the phylogeny of this frequently neglected group. The early history and views on the (pre-Hansen) classification of the Hydrophiloidea have been reviewed in detail by Hansen (1991, 1995). The intervening dozen years have seen more than ten subsequent papers addressing the interrelationships within lineages of Hydrophiloidea. Among these are the first works incorporating molecular data to infer the phylogeny.

Despite this plethora of studies involving both morphological and molecular data, the branching pattern among hydrophiloid families remains controversial. A summary of recent cladograms would result in an entirely unresolved strict consensus tree. Interestingly, despite the generation of numerous morphological and molecular datasets, none have been combined into a total-evidence framework. Consequently, each hypothesis is based on a particular character subset such as head structure (e.g., Beutel 1994, Anton & Beutel 2004), thoracic structure (Beutel & Komarek 2004), DNA sequence data (e.g., Bernhard et al. 2006), and larval data (Archangelsky 1998).

Placement of the Hydraenidae

The subfamilial placement of the Hydraenidae has, at certain times, been controversial. Originally a subgroup of Hydrophilidae, the lineage was later considered a distinct family of Hydrophiloidea (e.g., d’Orchymont 1916, Crowson 1950) and then as a family of the Staphylinoidea (e.g., Hansen 1991). Although the latter view has been generally accepted, it was recently challenged by Beutel (1994) who favored the former based on considerations of adult and larval head capsule structure. Recent morphological and molecular evidence uniformly supports the more accepted placement of the family Hydraenidae within the Staphylinoidea rather than the Hydrophiloidea (e.g., Korte et al. 2004, Beutel & Leschen 2005).

Nomenclature

Hydrophiloidea (s. l.) vs. Hydrophiloidea (s. str.)

The classification outlined here follows the Hydrophiloidea (s. str.) approach, which was standardized by Hansen (1991, 1995, 1999). Under the Hansen Classification, the term Hydrophiloidea excludes the Histeroidea, and the group is composed of 6 families. Although this classification is used in nearly all contemporary works (including all thirteen phylogenetic studies mentioned above!), there remains some residual use of “Hydrophiloidea (s. l.).”, in which the Hydrophiloidea encompasses the Histeroidea, and each of the six families of Hydrophiloidea (s. str.) is afforded subfamily status. To be sure, these two viewpoints are phylogenetically congruent, and differ only on ranks and circumscription. While the Hydrophiloidea (s. str.) approach is widely used and accepted by nearly all aquatic beetle authorities, a small number of publications still utilize Hydrophiloidea (s. l.) (e.g. American Beetles, see Van Tassell, 2001).

Bameul, F. 1994. New Oriental Oreomicrus Malcolm (Coleoptera: Hydrophilidae: Sphaeridiinae) with a redefinition of the genus and notes on its phylogenetic relationships. Annales de la Société entomologique de France 30: 79–91.

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Hydrophiloidea. Water scavenger beetles.
Authored by
Andrew Short.
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